Phase and frequency control



P 12, 1933- H. NYQUIST PHASE AND FREQUENCY CONTROL Filed March 11, 19313 Sheets-Sheet 2 Sept. 12, 1933. H NYQUlST PHASE AND FREQUENCY CONTROLFiled'March 11; 1931 3 Sheets-Sheet 5 Position, 22'

INVENTOR EJV 6 BY yguw ATTORN EY Patented Sept. 12, 1933 ATENT; orF cr.

1,926,169 PHASE AND FREQUENCY; CONTROL -Harry Nyquist, Millburn; N. 3.,'assig'nor to American Telephone and Telegraph Company, a corporation ofNew York Application March 11, 1931. Serial No. 521,851 1 21 Claims.(01. 250-2) This. invention relates to an arrangement for maintainingdistant sources of current substantially in phaseandfat approximatelythe'same frequency. One embodiment of the invention is described hereininconnection with an arrangement for controlling the phaseandfrequencyof the carrier waves of a chain of broadcasting stations.

When two or more distant radio stations are connected by telephone linesin order to broadcast the same program,; it is desirable that allbroadcasting stations'use the same carrier frequency so that thenumberof frequency channels required for one program may be a minimum.

'- Unfortunately, slight changes in. the relative phase of the radiofrequencyfcarrier currents from the various broadcasting stations afiectthe field strength in certain intermediate, localities,

thus causing received'signals to vary in amplitude and in seine cases tovanish completely.

If two radio broadcasting stations are to operate ,on the same nominalcarrier frequency, .11; is

' necessary, if the service furnished is to be satisfactory and at thesame time reasonably economi-- cal, that the nearness the actualfrequencies come tobeing'the same be considered with some care..It-would obviously be ideal, from the point of viewof the servicerendered, to requirethe actual'frequencies tobe identical. Ifthis' werethe case thejphase relation would be invariable and at any givenreceiving point there would be a fixed phase relation between thefields. received from the two broadcasting stations.

In practice, however, itwould be sufficient to merelyspecify that thephase angle between the carrier waves of the two stations should notchange by more than a certain number of degrees,

1 and to specify in addition that the rat'e of :phase it is not soimportant to limit the maximum phase drift should not exceed agivenjvalue. A slow phase drift merely causes gradual fading atthereceiver, which maybe taken care. ofby again adjustment in the receivingset itself. Rapid phase drift,thowever', would obviously. ruin thereception,'eitherby causingfading which is too rapid to, be compensatedfor at the receiver or by causing such a rapid fluctation .of the signalstrength thatthe signal would be distorted. It'is absolutely essential,therefore, that the rate of phase drift be limited. If, however, therate of phase drift be held within sufficiently small limits,

anglewhich may be permitted between the stations. 1

f It has heretofore been proposed to maintain the desired constancy offrequency and phase relaimportant.

transmitted over a line, the impulses at the rebe manifested as changedin'rphase.

tion by transmitting a control current from a master station over wirelines to the various radio stations. At each radio station the carrierwave from the station is generated by means of a local generatonsuch asa high frequency oscillator. 0 The frequency of this oscillator isautomatically adjusted whenever the normal relation between thegeneratedcarrier wave and the received control wave is changed more thana'given amount, and thisadjustment is automatically made regardless ofthe cause of the variation. 7

In such a system differences in the phase and I frequency of the carrierwaves of different radio stations will depend upon'two general sourcesoferror: (1-) errors due to the oscillators usedfor generating the carrierwaves at the radio stations or errors due to the oscillator used forgenerating the control wave at the master station, and (2) errors due tovariations in the transmission line used for transmitting the mastercontrol wave.

The errorsof oscillators are usually classified for convenience into (a)the longtime error in the rate or frequency of the oscillator and (b)the short period errors or differences in period of successive cycles.The' characteristics of oscillators of course vary, but inmost casesparticular attention is paid to reduce. the long timeerror because it iscumulative, and itis assumed that the short period errors are lessimportant becausein general they are not cumulative. However, the shorttime errors in some in-,- stances may be so .relatively'large as tobecome Where suchis the case, it is now possible, by the use of modernmethods, to so design crystal-controlled oscillators that both the longtime error-and the short time error may be kept verysmall. I

As to the errors due to the line itself, it should be noted that if acurrent of given frequency is ceiving end as compared with those at thesending end will, of course, show no long time error. Short time errorsmay occur, howevenand will These are due to-variations in the elementsgoing to .make up the line caused by such things as tempera--. turechanges, repeater steppings and (if this particular channel is amplifiedin repeaters common to other channels) overloading of -repeat. ers bysignals on other channels. Q 7 It should also be noted that the phaseshift be tween two oscillators which is caused by difference betweentheir long time frequencies, is cumulative with the first power of thetime. The

phase shift caused by short period errors, if these no are random, iscumulative with the square root of the time. The short period errorscausing phase shifts between the two ends of a line are not entirelyrandon'nhowever, so that although for a short time the phase shift mayaccumulate as the square root of the time, over a long period it willpractically accumulate to zero. In

other words, the phaseshift due to changes in.

the line will not accumulate over a long period but in general the phasewill be advanced or retarded withrespect to an average phase relation,and the amount of this phase advance or 7 phase retardation with respectto this average will not exceed a given maximum for a given line.

With these conditions in mind, it becomes-evident that a master controlsystem such as above outlined will suppress long; period diiferences infrequency by-not allowing the phase shift to accumulate. Also, the shortperiod difference.

between the resultant of the short period errors inthernaster oscillatorand the controlled oscillator, and the error-inthe line, is taken up byvarying the controlled oscillator. Such a method of control will besatisfactory where the re sultant of the short period errors in the twooscillators is of the same order of magnitude or greater'than the shortperiod errors in theline. However, if oscillators are available whichare constructed to be so steady in their operation that their shortperiod fluctuations are substantially negligible when compared with thecorresponding fluctuations in the line, this method forces thecontrolled oscillator to conform to the vagaries of the line andwastesthe valuable property of the steadiness in the oscillators.

In accordance with the present invention it isproposed to takeadvantagelof the'fact that the frequency of the oscillators may be heldwithin narrow limits so that the cumulative long period error may berelatively small as compared 7 with the error due to fluctuations in theline. To this end it'is proposedin effect, to loosen the couplingbetween the receiving end of the line "and thecontrolled oscillator atthe radio station so-lthat the oscillator will not be adjusted .in

response to the short period variations of the line but will be adjustedwhen long time variations occur.

This is accomplished by introducing between the line-and the controlelement a phase compensating device by which changes 'in'ph'ase withinlimits corresponding to the phase changes introduced by the line itselfwill'be cornpensated for without causing any adjustment of thecontrolled oscillator. When the phase change exceeds this limit,however, as will be the case when cumulative long time errorisintroduced, by anydiiference of the frequencies of the two oscillators,the mechanism for adjusting the oscillator will be brought intoplaytobring the frequency of the oscillator back to its normal relationto the control wave.

7 More specificallythe inventioninvolves-transmitting a' controlfrequency (preferably a voice frequency) from a master stationover aline to I the radio station and at'the radio station stepping 'up thecontrol frequency to a frequency equal to that of the local oscillatorfor generating the carrier wave of the radio station. Some of the energyfrom the local generator or oscillator at the radio station is appliedto a push-pull detector circuit, together with'energy from the telephoneline. The currents in the two output branches of the push-pull detectorwill be balanced at a certain optimum phaserelation of the two inputwaves. Any departure from this phase relation produces an unbalance inone of two senses, depending upon the direction of the phase shift.Thisprinciple is utilized to control two separate mechanisms one ofwhich is a phase changer which may be connected in the telephone lineand the other of which is a variable condenser or other devicefo'r'changing the frequency of the controlled oscillator. The controlexercised over these two devices is such that for small changes of phaseof the order of those produced by variations 'in the telephone lineitself, the phase changer may be operated to compensate therefor withoutcausing any change in the frequency of the oscillator. The variablecondenser of the oscillator is connected to the control mechanism by alost motion device to permit of this operation With this arrangement thephase shifter takes up the short shifts (except those so short as not tobe able to effect a change in the setting of the device) between thereceiving end of the line and the controlled oscillator on theassumption that the latter is more'steady. If these phaseshifts startaccumulating to an appreciable amount, however, an adjustment is made onthe frequency of the controlled oscillator on the assumption that it isthe latter whichisbeginning to drift.

Thisis accomplished by means of the lost motion device. The optimumadjustment of the lost motion device depends upon the relative errors-to be'expected-from the line. and the oscillators.-

If the line is bad the lost motion should be large;

if it is good the lost motion should be small.

if the adjustment of the lost motion device is such as to take care ofthe maximum phase shift due to the line, it is evident that anadjustment of-the controlled oscillator will never occur unlesscumulative drift due tochange in frequency of the oscillator, occurs.If, however, the adjustment of the lost motion device isnot sufiicientto take care of the maximum phase changes which may occur in the line,occasionally the oscillator will be adjusted'due to a large phase changein the line. This, however, will soon be corrected, 'as'the resultantcumulative drift in phase between the oscillator and-the controlfrequency will cause re-setting of the oscillator.

One more point should be noted. With this arrangement the maximum phasechange between any two radio stations may exceed the maximum phase shiftintroduced bythe line (which is cared for by the lost motion device).

This is due to the fact that the phasecompensator' is adjustedsimultaneously with the adjustment of the oscillatorin additionto theadjustment introduced'to take care of the line shift. The amount of thisadditional adjustment of the phase compensator may be kept small,however,

by introducing suitable gearing or other means so arranged that a'veryslight change in the control mechanism over and above that necessary totake up the lost motion will bring the adjustment of the oscillator backto normal.

The invention may now be more fully underphase changer and the mechanismfor adjusting the phase. changer and the variable condenser forfrequency control; Fig. 2 (a) is a sectional'view of part of theapparatus of Fig; 2; Fig.3 illustrates a the electricalelements of thefrequency changer;

Fig. 4 is a vector diagram corresponding to the arrangement of Fig. 3;Fig. 5 is adiagram similar to Fig. 3 but showing various positionsofadjustment'of the'phase controller; While Figs. 6, 7 and 8 are vectordiagrams corresponding to definite positions of thephase' controller ofFig. 5. r Figure 1 shows the essential features necessary formaintaining distance :sources of current inphase and at the samefrequency. In "these drawings an 'oscillator O, which may generateavoice frequency of, say, 1,000 cycles per second,

is connected to an harmonic generator l-lGi to step up they frequencytocorrespond with thewave length of a' local broadcasting station. ,In thecase assumed,.the frequency is 50,000 cycles per second, and a filter F1is'provided to select the desired harmonic from the -other' harmonics inthe output of theharmonic generator FIG-1. Some of theenergy of thecontrol wave from the oscillator O is also connected through parallelrepeating coils to various telephone linessuch as Ln leading toother5broadcastirig stations. The apparatus associated with the .distantterminal of line Ln only is shown, it being understood that theapparatus associated with the other lines will be similar. In the caseof the line Ln the 1,000 cycle control frequency-passes through a transformer into a selecting filter F'n and, is impressed upon an harmonic.generator HG-n to generate the carrier wave for the broadcasting stationassociated with the distant end 'of the line Ln. r

The carrier wave in this instancealso has a frequencyof 50,000 cyclesper second. Accord-- ingly, the harmonic of this frequency is selectedby means of the filter Fn and is passed through the phase shifter PSn,symbolicallyindicated, and

by means of a transformer Tn is applied tothe grid circuits of apush-pull detector circuit comprising detectors Dn and Dn whose outputcir cuits are oppositely connected through opposing windings of a polarrelay Rn. The carrier wave supplied to the broadcasting station isgenerated by means of a local oscillator On. In order that the phase andfrequency of this oscillator may be controlled with respect to othersimilar, oscillators at other broadcasting sta tions. associated with.the control station,= some of the energyfrom the output of theoscillatorOn is connected to the common branch of the grid circuitsof thepush-pulldetector-arrange.- ment through a transformer T 1 V The relationship ofthe various elements of the push-pull circuit is such thatwhenthe wavestransmitted through transformers-Tn and T'n are 90 degrees. out of phasewith respect to each other, equal opposing currents will flow throughthe two windings of the polar relay Rn and its armature. is held inneutral .posiiton. A phase shift of the two Waves in one directionresults in an increase in the direct current flowing in the output ofthe one detector and a decrease of thedirect current flowingin theoutput of the otherdetector, thereby producing an unbalance which causesthe polar relay to attract its arthe polar relay will attract bolicallyindicated at lEPn, whereby the phase shifter PSn may be adjusted and afrequency control arrangement such as the variable condenser associatedwith the oscillator On may be adjustable.

Thedetails of the mechanical parts MPn and the phase shifterPSn andtheir relationship are shown in Fig. 2. Before explainingthese details,

it is desirable, however/to have an understanding of what is required inorder that the circuit shall function properly. Bearing in mind that thepush-pull arrangement of Fig.1 is so arranged that a very smalldeparture from the proper phase relation of the two input waves resultsin an operation of the polar relay, and hence, of the control mechanism,it is desirable that some provision should be made whereby short timechanges in phase,.arising from abnormal conditions .(such as temperaturechanges, repeater steppings, repeater overloading, etc.) of the telephone line overv whichthe control frequency is transmitted, should becompensated for without adjusting the frequency control arrangement forthe oscillator On. Furthermore, since any change in'the frequency of theoscillator results in a cumulative phase shift of the carrier wave within response to such a' cumulative phase shift the oscillator On will beadjusted to bring its fre quency to normal. 1

These requirements are met in the present instance by connecting thephase shifter PSn so that it may be adjusted by the motor Mn whenever a.shift in phase occurs between the control wave and. the carrier waveregardless of the amount of said shift. ,The variable condenser Cn forcontrolling the frequency of the local oscillator On, however, isconnected to the motor drive through a lost motion device. This permitsthe. phase shifter PSn to be adjusted over a small w range tocompensate; for the small changes in phase which arise in the telephoneline itself without disturbing the frequency of the oscillator. When thefrequency of'the 'oscillator does change with the consequentcumulativeshifting in phase between the control wave and the carrierwave, the lost motion is taken up and the phase shifter'PSn and thevariable condenser Cn are adjusted together until the normal phasedifference between the control wave and the car-J rier wave is againestablished. When this occurs the frequency of the oscillator will beback to normal. As has been previously pointed out, the phase shift dueto the line is advanced or retarded with respect to an average phaserela-' tion. The lost motion is as proportional with respect to themaximum change of phase due to the line as to cause no adjustment of theoscillator in response to ordinary line changes. A phase change greaterthan that for which the loss of motion is" designed is assumed to be'dueto cumulative error in the oscillator and causes ad- J'ustment of thephase shifter PStuntilthe loss -with two armsb and b.

'g and h. which are normally 90 degrees apart,

as shown in Fig. 2a. 'Theshaft S is also provided A corresponding arm ais carried by a shaft S and a pin upon the arm aniay be engaged by. oneor the otherof the arms'b'and b when the shaft S is rotated through asuitable arc. The shaft S drives the variable condenser Cn (through asuitable reduction gear, if desired) and the arrangement of arms a--b bprovides a lost motion arrangement so that'the' cranks g and It may bedriven through a suitable range of adjustment without any change'of thevariable condenser Cn.

Two drive rods 0 and d are connected from the' crank throws g and h toinsulated portions of suitable sliders p-and (1 which move over a pairof guides c with 'a'travel approximately proportioned tothe sine andcosine of the angular positions of their respective crank throws. Thesesliders make contact with the uniformly Wound resistances 2R and alsowith the metallic guides which are connected to the terminals of theprimary of the input transformer Tn. The resistances 2R are connectedin'two parallel circuits bridged across the terminals of the telephoneline. In the series branch with the one resistance are two capacitiesX0, onearranged .on either side of the resistance. In series with theresistance in the other branch are two inductances XL, the inductancesbeing arranged on either side of'the resistance. By making XL;X0=R atthe frequency 1 of transmission, which in this case is50,000 cycles, theresultant voltage across the primary on the input transformer can beshown to have a substantially constant amplitude, with phase shifted ateither direction, depending upon the direction of rotation of the motorMn. As shown on the drawings, the sliders will pass each other moving inopposite'directions at approximately 0.707R from the middle of theresistance 2R when the connecting rods are long.

* In order to understand the operation of the phase. shifter PS1, let usrefer to Figs. 3 to 8, inclusive. Fig.3 shows the circuit arrangement ofthe phase shifter in simple form. If we imagine a voltageEo applied tothe terminals of the network in Fig. 3,v currents I1 and I2 will flowSince the elements R, X0 and XL have impedanceswhose numerical valuesare equal, the numerical values of the through. the branches.

currents I1 and I2 will be equal. If We consider the impedancesof'the'two capacities inthe one branch as being combined in one, and theimpedances of the two inductances in the other branch likewise combined,and represent the drops through the elements of each branch due to thecurrents I1 and I2 flowing therethrough,

respectively, the resultant vector diagram will i be as represented inFig. 4:. ,The vectorial drop through the capacities and resistance 'mustbe equal to the applied voltage E0. The drop 2 I1 R through theresistance. 2R will be degrees out of phase with the drop 2 I1 X0through the total capacity of the branch and the drop through theresistance will be numerically equal to that through the capacity of thebranch since the-impedances of the resistance and capacity arenumerically equal and the same current iiows through both. Applying thesame principles to thedrops through the resistance and inductance of thesecond branch, we get the two vectors 2 IZ'XL and 2 12 R.

With the mechanicalarrangement previously,

described the two contacts will be simultaneousdirection of movement ofthe contact as it approaches each'contact point. Corresponding contactpositions on the two resistances are indicated by the same numeral,primes being added in the case of the resistance in the right-handbranch. I

First, let us suppose'that the contacts are in positions 1 and 1. Let ussee what is the effective drop between these-two points. Referring tothevectorial diagram of Fig. 6, the vectorial drops through two branchesare as shown in full lines. Here the drops through each of the twocondensers are shown as separate vectors, and likewise, the dropsthrough each of the two inductances are shown by separate vectors. Theresult is that the applied voltage E0 instead of being subtended by twovectors for each branch so as to form two triangles as shown in Fig. 4,is subtended by three vectors in such manner that the resultant vectordiagram forms two pairs of triangles.

points 7 and 1 of the second branch. The drop between points 1 and 1will be the vectorial resultant of these drops; It will be noted thatthe circuit traced from the point 1 to the point 1 passesthrough theuppercondenser of the first branch in the direction opposite-to thedirection of current flow so thatin' taking the vectorial sum of thedrops, the dropthrough the condenser must be reversed- Asshown in fullline in Fig. 6, the vectorial drop through this condenser has the valueI1 Xc. Let us draw the drop throughthis condenser in the oppositedirection as shown by the dotted vector .-I1'Xc. The drop through theupper inductance of the second branch will be I2 X1, as shown in fullline inFig/e. If We obtain the resultant of this drop with the drop .-I1 Xofwe obtain the vector Va. It remains to combine this vector withthevectorial drop through the resistance R between the points ,7 and 1'.This drop through the resistance R will have a value of one-half of thevector 2 I2 R of Fig. 6 and will be in the same direction. Drawing thisvector in dotted lines at I2 R and obtaining the resultant of 50 thevector thus drawn'and the vector. Va, we have as the vectorial drop fromthe point 1 to the point 1 the dotted line vector V1. This vector, itwill be noted, is numerically {equal and in phase with various otherpositions which the two sliding con-.

tacts may assume ll'1'Flg. 5, it will be found that the resultant vectoris always of the same numerical value (that is, has the same length):but rotates about the'point O of Fig. 6. This will be 1 clear byconsidering two other positions ofthe sliding'contacts. i Let us firstconsider the case,

where'the sliding contacts are in positions 2 and 2', respectively,which are the next positions in order of rotation of the crank shaft.Due to the sine relation that exists between the motion of the cranksand the motion of the sliding-contacts, the

left-hand slidingcontact only passes over about .3 R in passing fromposition 1 to position 2. The

right-hand sliding contact, .on the other; hand, passes overapproximately .7 R. in going from point The method of obtaining thevectorial drop between points 2 and 2 is shown'in Fig. 7.] Here we havereverse drops through .3 Rbetween 2 -and .l and alsothrough the uppercondenser of the left-hand branch, these drops being in series withthenormal drops throughthe upper induct-y Y ance of the right-hand branch,through the resistance R and a resistance ofapproxim'ately .7 R, thelatter. resistance being between points 5' and 2. 1 First let us drawthe vectorial drop through theresistance .3 Has shown in dotted line.This drop .is oppositeginydirection to the drop 2 I1 R, in Fig. '7' andhas the value -3 11 R as shown in dotted lines. Since this vector isopvectors may be combined to give the resultant posite in direction tothe vector I2 XL representing. the drop through the upper inductance,these two vector Vb. Next let us draw the vector represent. ing the dropthrough the upper. condenser aswe did-in Fig. 6. Thisvector is shown at-11 Xo of Fig; '7, and whencombined with the vector Vb gives'a resultantvector Vd. There remains to be considered the drop throughthe resistancebetween -points '7 and 2. The vector representing this drop will be inthe same direction as the drop? I2 R inFig. '7 but its numerical valuewill be only 1.? I2 R and this vector is shown indotted lines at 1.7 I2R. of Fig. '7'. Combining this vector with the vector Va we obtain theresultant vector V2, which is indeed theresultant of all the vectorsconsidered. Numerically, the vector, V2 is equal'to the vector V1 ofFig. 6 but is rotated in phase degrees withrespect thereto, so thatas'the result of the 45-degree rotation of the crank shaft, there is ashifting in phase in the drop between the slidingcontact's of 45 degreesl without any change" in amplitude;

Now let us consider thedrop'for the last positions 8-8 of the twosliding contacts before coming back to their starting positions. It willbe I noted that the two contacts are now directly op posite each otherbut are moving in opposite directions .as shown by the arrows in Fig. 5.The drop betweenthese two points is the resultant of reverse dropsthroughyfi B, through the upper condenser, and normal drops through theupper inductance and the. resistance 3R in the righthand branch.

This is shown vectorially in Fig. 8. First let'us draw the vector. .3 l1R'for the reverse drop through the resistance in the right-hand branch.

"also :beclear that if the crank shaft is rotated continuously therewill be a continuousrotation This vector is oppositeindirection'to,thevector I2 XL for the upper inductance of the right-handbranch and upon combining these two vectors we get the resultant vectorV e shown in dotted lines.

Next, let us draw the reverse drop through the upper condenser of theleft-hand branch as shown at I1 Xo. Finally, let us draw the vector .3I2 R 'representingthe drop'throughthe resistance between 7 and 8" in therightvha-nd branch. This vector is opposite in direction to the ,vectorI1 Xe and when these two vectors arecombined we get, a resultant vectorVr. If we now combine vectors Ve and Vr'we obtain the resultant or shiftin phaseintroduced by the phase shifter With this understanding of theoperation of the phase shifter in mind, let us assume that'the arm a ofFig. .2 carrying the pin is in'some. position between the arms '2) and band thatgdue tosome cause such as change in temperature, the phase ofthe telephone line is shifted by a small amount. The output circuit ofthe push-pull detector arrangement will be unbalanced and relay R1 willset; the motor, M1 into rotation in one direction, thereby adjustingthe-:phase shifter PSn until a compensating .phase shift is introduced.into the line, thus bringing the two wave components entering thedetector again into proper relation, whereupon the armature of the polarrelay R11 is restored. to neutral position and the motor ceasesrotating. The wave entering the detector'from' the'line is now restoredto'its original phase. Obviously, small changes in phase carrierwavewithrespect to the control wave coming from the telephone line. :Againthe pushpull circuit is unbalanced and the motor. Mn is set intorotation. The actual working of. the mechanism will depend-upon theposition of the pin carried bythearm a. with respect to the. arms I) andb. ,Letuus suppose that 'atthe time the motor starts. to'rotate the-pincarried by the arm a is midway between the arms I) and b. As the'shaft Srotates,.the phase shifter may tend to momentarily bring about a balanceof. the push-pull circuit, the adjustment of the phase shifter tendingto compensate forv theaprier wave andithe control wave due tothe errorin the oscillator. Since where the two waves are parent existing phasedifference between the carnot at exactly thesame frequency,the effect ofthe error in the oscillator is cumulative, however,

the push-pullcircuit is immediately unbalanced 1 and the shaft Scontinues to rotate until the pin has been similar to its action wherethe difference the arm I) or b as the case may be,

carried by the arm a is brought into contact with V Unto this point theaction of the mechanism in phase was due to an error in the telephoneline itself. The action is one of attempting to" compensate by means ofthe phase shifter for the apparent-difference in; phase of the car-'rier wave and the control wave. As soon, however, as the pin comes incontact with the'arm b or b,-the cumulative phase difference due to theerror of the oscillator causes the shaft S to continue to rotate and theshaft S begins to rotate with the shaft S,'thereby bringing about anadjustment of the variablecondenserCn.

There is a simultaneous adjustment of the phaseshifter PS, and these twoactions continue together until a balance is again obtained. The

I result is that the frequency of the oscillator at the arm a restingagainst one of the arms 1) or the radio station has been changedslightly, tendto bring it back to the same frequency as the controlwave.- The result of this action is, of

, c'ourse,to bring about an adjustment of the phase shifter PSn, whichwas not necessary in order to bring the carrier wave back to the samefrequency as the control wave. This adjustment of; the

phase shifter cannot be avoided, however, be-

cause the "mechanism forv operating, the phase shifter cannotdistinguish between a phase differ'ence due to the telephone" line and acumulative phase difference due to an error in the'oscillator. w

As a consequence of thisadjustment J of the phase shifter, itwill benoted that the phase relation between the radio station at which thecondenser Onwas adjusted and any other radio station in the chainwill-to some extentbe a function of the correcting condenser. This' canbe tolerated, however, so long as the shift does not occur toofrequently, and it is assumed that a crystal-controlled oscillator ofthe type now known in the art, is good enough to avoid the necessity:of; adjusting 1 the correcting condenser too frequently. .After' theadjustment the mechanism has come to rest with the pin carried by b asthe case may be, and with the shaft S shifted an amount corresponding toonehalf the lost mo tion plus the additional amount necessary to bringabout the desired adjustment'of the correcting condenser 011., If, withthe apparatus in this condition', amaximum phase change should occur inthe line in suchdirection as to cause the shafts to rotate in the samedirection, both the phase 55.

again operated until the condition of balance-was shifter and thecorrecting condenser would be restored. Of course, the adjustment of thevariable condenser tends to assist the phase shifter in-compensat'ingfor' the phase shift in the 6 u line, but the adjustment of thecondenserresults in an over-correction of the frequency of theoscillator... This'will result ina cumulative phase differencebetween'the carrier wave and the control wavein the opposite sense tothat which caused the previous correction of the oscillator, and theshaft S will: becaused 'to-rotateiin the opposite direction untilthelost motion'is taken up in the opposite direction and the variablecon-i denser and phase shifter simultaneously adjusted until thefrequency of the oscillator is corrected.

The mechanism will now come to rest with the pinch the arm a restingagainst the other of the? arms bor b as the case may be. 'lf,in thiscondition a maximum phase shift'in a direction oppo site to thatpreviously considered should'occur in I Y 1,926,169 a i V the lineitself the push-pull circuit will be again unbalancedandthe shaft Swill'be rotated in the same direction as it was last moved until theadjustment of the variable condenser ,Cn and the phase shifter'PSncompensate for the phase shift in'the line. This would also result inover-correctionof the variable condenser in the opposite sense to theformer over-correction, so that a cumulative phasediiference wouldultimately result which would cause the mechanism to correct the erroras before.

Of course when the pin carried by the arm a lies in an intermediateposition between the arms band b any phase shift in the line can becorrected by adjustment of the phase shifter PS1 so long as thephasechanges in the line (which compensate to zero over along-period) do notrequire the pin carrled'by the arm a to move out of the angle betweenarms 1) and b. p 1

It will be apparentfrem the foregoing description, that the'phaserelation ofthe carrier wavev of the'station illustrated with respect tothat of the carrier wave of another station in the chain,

may shift by an amount corresponding toa rotation of the shaftSsufficient to take up the complete loss of motion between the arms byand 1) plus twice the additional rotation of the shaft S necessary forthe variablescondenser and phasesary to rotate the shaft S over andabove the amount required to-take up theloss of motion, in order to keepthev possible phase difference between the station in question and anyother 'staion in the system as small as possible. This may beaccomplished by using a gearing to control the operation of the variablecondenser On, this gearing being so arranged that the maxi'mumadjustmentof the variable. condenser necessary to compensate for any phase changeWhich brings it into play, will be accompanied by a simultaneousadjustment of the phase shifter PS, which'is very small as compared withthe adjustment of the phase shifter represented by a movement of theshaft sufii'cient to take up the complete loss of motion. The result isthat the maximum of adjustment of the phase shifter which 'couldnormally be expected to occur would be but little greater than theadjustment corresponding to a movement of the shaft S sufficient totakeup the complete loss of motion between the arms bandbfl Since themaximum adjustment of the phase shifter PSI; in any given direction"from its normal position'is a measure of the actual shift in phase ofthe generated carrier wave of the station with respect to the controloscillator, the carrier wave ofthe station can only get out of phasewith respect to the carrier wave of any other 1 station whose phaserelation has not changed, by an amount equivalent to this maximumadjustment of the phase shifter, since the 1 other station may bepresumed to be in normal synchronous relation with respect to the commoncontrol oscillator. Now the maximum possible adjustment which maybeexpected of the phase shifter PSn under any condition. beingbut littlecorresponding to one-half the angle greater than the angle 1 between thearms b and b, it becomes evident that even if another station in thechain should have its adjusting apparatus adjusted to the maximum phase,shift of its phase shifter in one direction, and the station hereillustrated should haveits phase shifter adjusted the maximum amount inthe opposite direction, the two stations would only be ou'trof phase byan amount equal to the angle between thearms b and la plus twice theadditional rotation of the shaft in either direction that would benecessary for the maximum expected adjustment of the condenser On.

-In other words, ifboth stations had their carrierwaves in normalphaserelation; with the pin carried by the arm a at both stations justhalf- Way between the arms b and b", an error causing one of t estations to move its shaft S to'its maximum expected adjustment in onedirection would merely adjust its phase shifter anamount etween the arms22 and b, plus the additional angle in the same direction correspondingto the maximum adjustment of the variable condenser Cu at that station.If, at the same time an equal but opposite error should occur. at theother station, causing its apparatus ,tobe adjusted in the oppositedirection, the shaft S at the latter station would a be adjusted in theopposite direction by an amountequal to one-half -the angle between the"arms I) andb? plus the additional angle in the same directioncorresponding to the maximum adjustment of the condenser On.

From the foregoing it will be seen that'the ar rangement of theinvention not only tends to keep the'carrier waves of; all of-thestations of the chain at the'same frequencybut it limits the maximumamount that the carrier waves-of any two stations inayget out ofphasewith respect to each other By means of the provision of the lost motiondevice, it provides for a compensat ing adjustment of thephaseshifter-alone with H out any necessary adjustment of the variablecondenser controlling the oscillator, where there has been-a change inphase due entirely to the line transmitting the controlfrequency. Thisrenders it unnecessary to make the. highly stable oscillator whichgenerates the carrier wave follow the, relatively large phase'variationsdue to the line itself. I l

t will be obvious that the general principles hereindisclosed may beembodied in many other organizations widely difierent from thoseillustrated without departing from the, spirit of the invention asdefinedin the following claims.

What is claimed is:. Y I g 1; In a frequency control system, means togenerate a wave whose frequency -is to be controlled, means to supply acontrol wave, relaymeans responsive to certain changes in thenormalphase relation of the control and controlled Waves, means controlled .hysaid relay means to adjust the frequency ofjthe controlled'wave inresponse to'sueh changes; and means to defeat such adjustment when saidrelay means oper-. ates, in response to phase changes of the controllingwave only. "1 q 2. Ina frequency control system, means to generate awave whose frequency is to be controlled, means ,to supply a controlwave, relay means responsive to'certain changes in the normal phaserelation of the controland controlled waves, means controlled by saidrelay means to adjustthe frequency of the controlled wave in response tosuch changes", and means to defeat suchadjustrnent when said relay meansoperates in response to small changes from the normal phase relation ofthe control and controlled waves.

3. ma frequency control system, means to generate awave whosefrequencyis to be controlled, means to supply a control wave, a push--pull detector to which both waves may be applied, meanscontrolledbysaid detector to adquency in response to phase'changes of the con-.

trolling wave only. I I

A. 1111a frequency control system, means to generate a wave whosefrequency isto be 0on -v trolled, means to supply acontrol wave,apushpull detector to which both Waves may be ap-. plied, meanscontrolled by said detector to adjust the frequency of said control.Wave in response. to certain changes from the normal phase relation ofthe control and. controlled waves applied to said detector, and meanscontrolledby said detector to prevent adjustment ofv frequency inresponse to small changes from the normal phase relation ofv the controland controlled wave. V r

5. In a frequency control system, means at a in response to certainchanges in the phase rela-' tion of the waves applied to saidcontrolling device,v and. means to defeat such adjustment when saidrelay operates in response to phase changes in said line.

, 6. In a-frequency control system, means at'a station to generate aWave whose frequency is to be controlled, means to generate a control:

wave atla distantv point, aline to, transmit said control wave from saiddistant point to said s'tation, a push-pull detector at said station towhich said control and controlled wave may beapplied, means con rolledthereby to adjust the frequency of said;contro1led wave in response 'tocertain changes in the phase relation of the waves applied to saiddetector, and means to prevent such adjustment in response to phasechanges in said: line.

7. In a frequency control system, means at a station to generate a wavewhose frequency is to be-controlled, means to generate a control wave ata distant point, a line to transmit said control wave from said distantpoint to'said station, a controlling deviceat said station to which saidcontrol and controlled waves maybe applied,

means controlled thereby to adjust the frequency ofsaid controlled wavein" response to certain changes in the phase relation of ,thewavesapplied to said controlling device, a phase changer in said line, saidphase changer being controlled by said controlling device, theconnections between said controlling device and said frequency adjustingmeans and said phasechangerbeing: such that in response to phase changesin said line said phase changer will be adjusted to compensate thereforwithout operating said frequency adjusting means.

8. Inafrequency control system, means-at a,

station to generate a wave whose frequency is to be controlled means togenerate a control wave at a distant point, a line to transmit saidcontrol wave from said distant'point to said station, apush-pull-detector at said station to which said 7 control andcontrolled waves may be applied,

, as 'ofjsaidrelay, an adjustable, phase shifter in said I telephoneline, a frequency controller associated push-pulljdetectorat saidstation, means toapply andrsaidfrequencyadjustingw means and said phasechangerbeingsuch 'that'in response to phase changes,in'said 'line saidphase changer will be adjusted to compensate therefor without operatingsai'd'fre'quency adjusting means.

9.- In a frequency control system,means at a station to generate a' waveWhose frequency is to be controlled," means to. generate a control aveat a distant point, a line to transmit said :control wave from saiddistant point to said sta;

, I tion; a push-pull detector at said station, means to apply saidcontrol and controlled waves to .1

said detector so that they will be differential with respect to the oneand cumulative with respect to the'other unit oi said push-pulldetector,

; a polarnrelay having an armature, said relay n being so connected'tothe outputof said detector as to be neutral when the two waves areapplied to said detector in quarter phase relation but to attract itsarmatureto one side or the other depending upon the direction in whichsaid two waves depart ,from quarter phase relation, an operatingmechanism controlled by the armature iththe generator whose Wave is tobe controlled,

A andmeans operatively connecting said operating mechanism to said phaseshifter and said frequency controller. V i I 1 10. In afrequency controlsystem, means at a station to generate a wave whose frequency-is to becontrolled, means to 'generate a control wave at a distant -point,-aline to transmit saidcontrol Wavefrom said distant point to said,station, a.

said control and controlled wave to said detector so that they will bedifferential with respect to the oneand cumulative with respecttotheother unit-of said-push-pull detector, a polarjrelay having anarmature, said relay being so cona nected to the output of said detectoras to be frequency controller;

neutral when'the twowaves are applied to said detector in quarter phaserelation but to attract:

its armature to one side or the other depending upon the directioninwhich said two waves depart from quart'er phase relation, an operatingj mechanism controlled by the armature of said relay an adjustable phaseshifter in said tele-' phone line,'a frequency controller associatedwith the generator whose wave is to be controlled, and

' 'means op eratively connecting said operating mechanism,to'said phaseshifter and'said frequency-controller, said last mentioned means be--ing 'so arranged as topermit limitedadjustment of said phase changerwithout, operating said llf'Infrequency control system, a phase changer,a frequency controller, a drive shaft,

connections from said shaft'for' directly driving said phasechanger,'and a lost-motion drive between said shaft and said frequencycontrollen 12. In a frequency control. system, a phase changer, afrequency-controller, a driveshaft,

connections from said shaft for directly driving said phase changer, andmeans connectingsaid shaft and said frequency controller, said -meansincluding means to delay the operation of said frequency controllerafterthe beginning of rotation of said shaft.

13, In a frequency control system, means to generate a wave whosefrequency is to be controlled, means to supply a control wave, a relayjointly controlled by said control wave and controlled. wave meansoperated by said relay to determine the frequency of said controlledwave, and means to defeat a change in frequency of the controlled wavewhen said relay operates in response to certain changes in the arrivingphase of the controlling wave.

i l. In a frequency control system, a controlling source of cyclicvibrations, a controlled source of cyclic vibrations, each of saidsources having small short period errors, a transmission mediumconnecting said sources; said medium having relatively large shortperiod errors but substantially no long'period error, and meansdiscrimi-' substantially no 'long period error, means discriminatingbetween short period errors and long period errors for associating saidtransmission medium and said controlled source to prevent the phaseangle of said controlled source from "varying more than a predeterminedamount, and

for preventing the rate of phase drift from exceeding a predeterminedvalue, and a phase compensator associated with said discriminating meansand controlled thereby'to compensatefor short period errors in saidmedium.

16. In a frequency control system, a controlling source of cyclicvibrations, a controlled source of cyclic vibrations, a transmissionmedium connecting said sources, said medium hav-,- ing short time errorsbut substantially no long time error, and said controlled source havingcumulative long time error, and means discriminating between'shortperiod errors and cumulative long period errors for associating saidtransmission medium and said controlled source to prevent the phaseangle of said controlled source from varying more than a predeterminedamount,

and for preventing the rate of phase drift from exceeding apredetermined value.

' 1'7. In a frequency control'system, a controlling source of cyclicvibrations, -'a controlled 'sourc e of cyclic vibrations, atransmissionmediumrconnecting said sources, said medium having short time errors butsubstantially no long time error,han'd said controlled source havingcumulative longtime error, means discriminating between short perioderrors and cumulative long period errors, said means being interposedbetween said transmission medium and said controlled source to preventthe phase angle of said controlled source from varying more thanpredetermined amount and for preventing the rate of phase" drift fromexceeding a predetermined value, and a phase compensator associated withsaid discriminating means and controlled thereby to compensate for shortperiod errors in saidmedium. a

18. In a frequency control system, a controlling source of cyclicvibrations, a controlled source of cyclic vibrations, a transmissionmedium connecting said sources, said transmission medium havingsubstantially no long time error and havingfshort time errorscumulatingpractically to zero over a comparatively long; period of time, saidcontrolled sourcehaving errors'cumulating as the firstpower of time, andmeans disc'riminating between short time errors cumulating to zero anderrors cumulating as the first power .of

time for associating ,said transmission medium and said controlledsource to adjust the'lfrequency of said controlled source; said meanspreventing any adjustment of the frequency unless the phase driftaccumulates to a predetermined angle.

19. In a frequency control system, a controlling f;

" source of cyclic vibrations, a controlled source of cyclic vibrations,a transmission medium connecting said sources, said transmission mediumhaving substantiallyrno long time error and hav ing short time errorscumulating practically to zero over ,a comparatively long period oftime, said controlled source having errors cumulating as the first powerof time, a phase shifter to compensate for short time errorscompensating the phase drift accumulates to an appreciable angle.

e 20. In a frequency control system, means to generate a wave whosefrequency is to becontrolled, means to supply a control wave, meansresponsive to certain changes in the normal phase relation of thecontrol and controlled waves to adjust the frequency of the controlledwave and readjust the phase relation between the control and controlledwaves, and means to defeat said frequency adjustment while permittingsaid' phase adjustment in response to phase changes of the controllingwave only.

21. In a frequency control system, means to: generate a wavewhosefrequency is to be controlled, means to supply a control wave,means responsive to certain changes in the normal phase relation of thecontrol and controlled waves'to adjust the frequency of the controlledwave and to readjust'the phase relationof the control and controlledwaves,'and means to "defeat said frequency adjustment while permittingphase adjustment in response to small changes from the normal phaserelation of the control and controlled waves.

, HARRY NYQUIST.

to zero, a frequency adjuster to adjust the fre-

